1. Field of the Invention
This application relates to oscillator circuits and more particularly to compensating for the effect of variations in the supply voltage on oscillator output.
2. Description of the Related Art
Voltage controlled oscillators are commonly found in such applications as phase-locked loop circuits. FIG. 1 illustrates a high level block diagram of a phase-locked loop 100. In the phase-locked loop 100 an input signal 101 and a feedback signal 103 are coupled to a phase/frequency detector 105 that determines the phase/frequency difference between the input signal 101 and the feedback signal 103. That difference is supplied to a loop filter 104, which supplies a control voltage on node 118 to vary the output of the voltage controlled oscillator (VCO) 110 in accordance with the control voltage. The voltage controlled oscillator (VCO) may be implemented using an LC tank circuit in which the oscillating frequency of the VCO is proportional to √{square root over (1/LC)}. It is common to vary the capacitance (C) to change the output frequency of the VCO.
When the supply voltage for the VCO changes as a result of, e.g., transient noise on the supply voltage, the output of the VCO can change. Such a change in VCO output is referred to as “supply pushing”. Supply pushing for a VCO is defined as an oscillation frequency change resulting from a supply voltage change (Δf/Δsupply voltage). If a VCO has large pushing, the oscillation frequency is sensitive to supply movement. In such cases supply noise directly translates into frequency change. Frequency change in turn integrates into phase change. When a VCO is sensitive to noise on the supply voltage, a noise profile on the supply voltage translates directly to the noise profile of the VCO output. VCO Single Sideband (SSB) phase noise (dBc/Hz) due to supply noise can be calculated as
      10    ⁢                  ⁢    log    ⁢                            Kp          2                ×                  V          2                ⁢        rms                    2        ⁢                  f          2                      ⁢          (              dB        ⁢                                  ⁢                  c          /          Hz                    )        ,, where the supply noise is assumed to have an rms amplitude of Vrms and a frequency off and Kp is the supply pushing in Hz/V.
Many specifications for communication modules allow at least some noise on the supply voltage. For example, the supply voltage for a board may be allowed to vary 10 mV peak to peak across frequencies. That means the integrated circuits on the board should be able to reject that noise at any frequency. Some specifications are even more difficult to meet. For example, the 10-Gigabit Serial Interface Module Group (XFP) specification allows peak to peak supply voltage variation by as much as 3%.
One possible approach to deal with power supply variations is to filter the power supply output. However, an RC filter would tend to drop the power supply voltage, which is undesirable particularly at the low voltage power supplies typical today. An LC filter could be used instead but only filters at certain frequencies.
Another solution is to have a voltage regulator on chip. In fact multiple regulators may be utilized. For example, a first regulator may be used to provide 20 dB of rejection and a second cascaded regulator can be used to increase the rejection to 40 dB. In the case of a 10 mV peak to peak power supply variation a 10 mV swing reaches the VCO 40 dB down. VCO regulators have to be stable and have low noise generation because of pushing. So the VCO regulator has to adequately reject supply noise (therefore two stages may be utilized) and its own noise has to be very small. Otherwise, whatever noise is on the regulator output is provided as part of the VCO supply voltage and will translate into phase noise if the VCO is susceptible to pushing. That can be particularly true in certain LC oscillator designs.